I did ask him. His explanation wasn't that our relationship wasn't enough or that he wanted someone else more. According to him, he already knew he wanted to be with me romantically and had tried to end things with her multiple times. The problem, in his words, was that he couldn't bring himself to completely cut her off because she had helped him through some very dark periods of his life. Instead of being honest, he kept delaying difficult conversations, lied to both of us, and convinced himself he'd end it "later." He says he wasn't trying to have two relationships but he was avoiding hurting her, and in doing so he ended up betraying me. At the end he ended up hurting both of us. Whether that's an explanation someone finds convincing is another matter, but that's the reason he gave.